1. Field of the Invention
The invention relates to a method of producing microscale cold cathodes, and more particularly, to an improved method of producing metallic microscale cold cathodes by which emitter cones for emitting electrons can be reproducibly and stably produced in given shapes.
2. Description of the Related Art
Microscale cold cathodes are essential components of emitting electrons for vacuum microelectronic devices such as extreme microscale microwave vacuum tubes and flat-panel display elements. The microscale cold cathodes are composed of, for example, an emitter tip having a conical shape formed on a substrate such as a semiconductor. The cone of the emitter tip is surrounded by a gate electrode, which is separated from the substrate by a gate insulating film, and a gate electrode aperture is formed in the gate electrode around the conical emitter tip. The principal parameters dominating the performance characteristics of the microscale cold cathodes are the radius of the aperture of the gate electrode, the height of the emitter chip, and the thickness of the gate insulating film, and the like. Also, the radius of curvature of the end of the emitter chip is a very important factor in the performance of a cold electrode.
Microscale cold cathodes having such a structure, known as Spindt-type cold cathodes, may be formed by a method using a leaning evaporation as described in C. A. Spindt, J. Appl. Phys., 39 (1968) p. 3504, or a method using a side etching as described in H. F. Gray and G. J. Campisi, Mat. Res. Soc. Symp. Proc., 76 (1987) p. 25. The former method is used when forming a cold cathode of metal, and the latter method is used when producing a cold cathode of silicon.
According to the method of Gray et al., a microscale cold cathode of silicon is produced as follows:
A first insulation film, e.g., a film of SiO.sub.2, having a uniform thickness is formed on a silicon substrate by a known thermal oxidation process, and thereafter a photolithography process is used to form an insulation film mask pattern having, e.g., a circular configuration, by etching the film with hydrofluoric acid. The thus-processed substrate is then subjected to a chemical etching process, e.g., with a KOH solution to anisotropically etch the silicon and form a cone beneath the insulating mask pattern. In this case, the etching process is stopped before the insulation film mask pattern is separated from the top of the cone.
A second insulation film, e.g., a film of Si0.sub.2, is then formed on the substrate from above, by an electron beam evaporation, in such a manner that a certain space is formed around the cone. Then, a gate electrode film, e.g., a film of Mo, is uniformly deposited on the thus-processed substrate from above by a known process, in such a manner that at least a portion of the side of the mask pattern of insulation film situated over the cone is exposed.
The mask pattern of the SiO.sub.2 insulation film is then etched with hydrofluoric acid (HF) to communicate the space around the cone with the external space thereof. In this case, the etching process is stopped at a point such that the mask pattern remains on the top of the cone. Thereafter, only the silicon is isotropically etched, by a mixed solution of HF and HNO.sub.3, to sharpen the end of the cone while separating the mask pattern from the cone, to thus form a microscale cold cathode having a silicon emitter tip on the silicon substrate. The configuration of the gate electrode is then adjusted by a pattern etching of the gate electrode film, as required.
In this method, however, it is difficult to reproducibly form emitter tips because of the difficulty of determining the point at which the etching should be stopped
An alternative method has been proposed, in which the etching of the silicon cone is stopped when the mask pattern of the insulating film is separated from the cone, and an ion beam of, e.g., Ar.sup.+ is irradiated to the plane top remaining on the end of the cone, to thereby remove the material around the center of the plane top of the cone and taper the cone end, and thus form an emitter tip having a stable and sharp end.
Although this method provides an excellent reproducibility, it has a defect of a poor electron emission due to damage caused by the irradiation of the ion beam.
Since silicon has a relatively high resistivity, sometimes silicon cathodes cannot be used in applications requiring a large amount of electrical current. Therefore, in such a case, it is necessary to use a metal having a high melting point and low resistivity for the emitter tip.
Cold cathodes of metal may be produced by the method described in the report by Spindt, as referred to above. According to this method, an insulation film and a gate film are sequentially deposited on a substrate, and an aperture is made through both films by an etching thereof. A material such as alumina is then obliquely evaporated, as a sacrificial layer, onto the surface of the gate film, while rotating the substrate, in such a manner that the evaporated material is not deposited at the bottom of the aperture. Thereafter, a metal material for the emitter is evaporated perpendicular to the substrate, whereby a conical emitter tip is formed inside the aperture and on the substrate due to a reduction of the size of the aperture in the gate film caused by the evaporation. Unnecessary metal is then removed by etching the sacrificial layer, to thereby complete the forming of a microscale cold electrode.
The end of the emitter tip thus formed has a radius of curvature at best of around 20 to 30 nanometers, and to obtain better electron emission properties, preferably the end of the metallic emitter tip has a smaller radius of curvature.